FY 1996

Impact of dimethylsulfide photochemistry on methyl sulfur cycling in the equatorial Pacific Ocean

Kieber, D.J., J. Jiao, R.P. Kiene, and T.S. Bates

J. Geophys. Res., 101(C2), 3715–3722 (1996)

Shipboard experiments were conducted in the equatorial Pacific Ocean to ascertain the relative importance of atmospheric ventilation, biological consumption, and photolysis in the removal of dimethylsulfide (DMS) from seawater. Comparisons were made at a series of sampling locations in a transect from 12°N 140°W to 12°S 135°W, as part of the International Global Atmospheric Chemistry project's Marine Aerosol and Gas Exchange cruise in February-March 1992. Turnover rate constants for DMS were used to compare the different removal pathways over three depth intervals (0-1 m, 0-20 m, and 0-60 m). In the surface mixed layer (0-60 m) the DMS turnover rate constants ranged from 0.02 to 0.19 day for atmospheric ventilation, 0.04 to 0.66 day for biological consumption, and 0.05 to 0.15 day for photolysis. When all three processes are considered, the corresponding turnover time for DMS ranges from 1 to 4 days, with photolysis accounting for 7%-40% of the total turnover of DMS. Laboratory irradiations were conducted with stored seawater samples to study the kinetics and wavelength dependence of DMS photolysis. Salient results were (1) the photolysis of DMS followed pseudo first-order kinetics, (2) dimethylsulfoxide was a minor (14%) product of DMS photolysis, and (3) the photolysis of DMS in seawater under natural light conditions occurred primarily at wavelengths between 380 and 460 nm. On the basis of these results, we predict that the photolysis of DMS will occur at appreciable depths in the photic zone in oligotrophic marine environments (~60 m). An important finding of this study is that atmospheric loss, biological consumption, and photolysis are all important removal pathways for DMS in the photic zone of the equatorial Pacific Ocean. The relative importance of each pathway is a function of the depth interval considered, sampling location, and meteorological conditions.